1. Field of the Invention
The present invention relates to a misfire detection apparatus and a misfire detection method for an internal combustion engine, which are applicable to an internal combustion engine including a plurality of cylinders, for determining presence/absence of misfire in each of the plurality of cylinders.
2. Description of the Related Art
In recent years, detection of a misfire state in an internal combustion engine (hereinafter also referred to as engine) has been demanded as environmental measures in order to prevent unburned gas from being exhausted, and to protect a catalyst provided on an exhaust system of the engine on four-wheeled vehicles as well as two-wheeled vehicles.
On this occasion, as a first measure for detecting the misfire state of the engine, there is a measure for monitoring a change in an r.p.m. of an engine (r.p.m. of a crankshaft), and detecting a misfire state of the engine based on the fluctuation in rotation (for example, refer to Japanese Patent Application Laid-open No. Sho 58-19532).
Moreover, as a second measure for detecting the misfire state of the engine, there is a measure for detecting the misfire state of the engine based on a rich/lean cycle of a detection signal from an air/fuel ratio sensor for detecting a mixing ratio of air to fuel of a mixture to be combusted in the engine, namely an air/fuel ratio (for example, refer to Japanese Patent No. 5208289).
However, the related arts have the following problems.
When the related art described in Japanese Patent Application Laid-open No. Sho 58-19532 is applied to a vehicle small in number of cylinders such as a V-twin engine and small in rotational inertia due to a small flywheel (such as a two-wheeled vehicle), fluctuation in rotation is significant, and the misfire state of the engine cannot be correctly detected. In other words, fluctuation in rotation in a normal state without misfire is increased due to influence of a difference in combustion state generated between cylinders, and a determination error in misfire and a loss in detection of the misfire thus occur, resulting in incorrect detection of the misfire state of the engine.
Moreover, in the related art described in Japanese Patent No. 5208289, the air/fuel ratio sensor is not provided for each of the exhaust pipes connected to the plurality of cylinders of the engine, but for a collector of the exhaust pipes. However, the air/fuel ratio sensor may need to be provided at a location spaced apart from the collector depending on a vehicle to which the related art is applied. In this case, the unburned gas exhausted in the misfire state and the burned gas exhausted in the normal state reach a detection area of the air/fuel ratio sensor in a mixed manner, and a difference in rich/lean cycle between the misfire state and the normal state may not be correctly detected. Thus, the misfire state of the engine may not be correctly detected based on the rich/lean cycle of the detection signal from the air/fuel ratio sensor.
Moreover, in the related art described in Japanese Patent No. 5208289, when lengths from the plurality of cylinders of the engine to the collector of the exhaust pipes are different from each other, the burned gas and the unburned gas exhausted from the respective cylinders reach the detection area of the air/fuel ratio sensor at timings different from each other, and the difference in rich/lean cycle between the misfire state and the normal state may not be correctly detected.
Moreover, in the related art described in Japanese Patent No. 5208289, the misfire state is detected only for closed loop control (also referred to as O2 feedback) in the air/fuel ratio control using the air/fuel ratio sensor. Moreover, in general, the fuel is gently increased and decreased in correction without increasing and decreasing the fuel by an amount causing a clear rich or lean state in order to alleviate influence on the engine behavior even for the O2 feedback control. Thus, there often occurs such a case that the value of the rich/lean cycle even after a normal combustion is not an intended value due to influence of fluctuation in the combustion states of the respective cylinders. Therefore, the rich/lean cycle in the misfire state and that in the normal state cannot be distinguished from each other, and these states may not be correctly determined.